1. Technical Field of the Invention
This invention relates generally to communication systems and more particularly to digital subscriber line (DSL) based communication systems.
2. Description of Related Art
Communication systems are known to enable a plurality of communication devices to communicate among themselves and with communication devices in other communication systems. Such communication devices, which may be computers, modems, facsimile machines, printers, personal digital assistants, et cetera, communicate voice, text, and/or video data. Such communication systems support the communication of data in accordance with one or more communication standards. As is known, there are a large number of communication standards for the communication of data and such standards vary from country to country. For example, there are a variety of standards governing digital subscriber line (DSL) communications and such standards vary from country to country.
As is further known, for a communication device to communicate via a DSL based system, the communication device includes a DSL modem. Typically, the location of the communication device with its associated DSL modem is referred to as the customer premises. The DSL modem at the customer premises is typically coupled via a twisted pair to a DSL modem at a central office. FIG. 1 illustrates an example of a DSL modem at the customer premise (CPE) coupled to a DSL modem at the central office (CO). The coupling is achieved via a twisted pair, which supports one DSL channel, and is one of a plurality of twisted pairs in a cable binder, or bundle of wires. In this example, the frequency allocation of the DSL channel is illustrated in FIG. 2.
As shown in FIG. 2, the DSL channel includes 4 frequency bands (band 1 through band 4). Each band may be allocated for upstream transmission (i.e., from the CPE to the CO) or downstream transmission (i.e., from the CO to the CPE). For example, bands 1 and 3 may be used for upstream transmissions while bands 2 and 4 are used for downstream transmissions. The width (i.e., frequency) and height (i.e., power) of each band may vary and are typically defined by one or more standards. For example, various DSL standards prescribe a frequency, or spectral, plan that define the transmit frequencies (i.e., start frequency and width) and associated powers (i.e., height) for each band. This is done primarily to minimize near-end-cross-talk between twisted pairs within a cable binder by having each twisted pair within a cable binder using the same frequency plan.
To support the DSL channel illustrated in FIG. 2, the CO modem and CPE modem of FIG. 1 each include two transmitters and two receivers. In addition, each modem includes a hybrid, which performs a 2-wire to 4-wire conversion, a summer, and a splitting multiplexer and a reconstruction multiplexer. Accordingly, for the example given where bands 1 and 3 are used for upstream data communications, the 1st transmitter of the CPE modem transmits the data in band 1 and the 2nd transmitter of the CPE modem transmits the data associated with band 3. The transmitters in the CO modem transmit the data in band 2 and data in band 4, respectively. Correspondingly, the receivers in the CPE modem receive the data in band 2 and band 4, respectively. Similarly, the receivers in the CO modem receive the data in band 1 and band 3, respectively. Alternatively, bands 1 and 3 may be used for downstream transmissions and bands 2 and 4 may be used for upstream transmissions.
The splitting multiplexers in the CO modem and CPE modem split the incoming transmit data between the respective transmitters. Conversely, the reconstructing multiplexers, reconstruct the data received from the respective receivers into a serial data stream.
When data can be allocated into all 4 bands, the CPE modem and CO modem are capable of transceiving data at a relatively high bit rate (e.g., greater than 5 Mbps). Typically, the shorter the twisted pair, the less cable loss and the less cross-talk the twisted pair, or loop, exhibits. Conversely, the cable loss and cross-talk increase as the length of the loop increases. When the cable loss and cross-talk increase to significant levels, the upper frequency bands (e.g., band 3 and band 4), become unusable. Thus, CPE modems coupled to the central office via shorter loops typically have higher bit rates than CPE modems coupled to the central office via longer loops. This creates a discontinuity in quality of service since some users have a higher bit rate than others.
Therefore, a need exists for a method and apparatus for a configurable modem that achieves high bit rates in a DSL system regardless of the loop length.